This invention relates to a heated roll fixing device for use in an electrophotographic copying machine.
Conventionally, a fixing device for electrophotographic copying machines which has been widely used is a heated roll fixing device.
Such a heated roll fixing device, for example, is of the type, as shown in FIG. 1, in which there are provided a heated roll 4 which is provided with a heating element such as infrared ray lamp which serves as an inner heating means 3 on the shaft portion of the roll, which roll is coated around a core 1 made of copper or aluminum with heat-resistant surface lubricants such as tetrafluoroethylene resin, silicone rubber, fluorocarbon resin or the like, and a pressure roll 5 which is rotated in cooperation with said heated roll 4. A supporting body 7 carrying unfixed thermoplastic powders (hereinafter referred to as "toner") in image configuration is passed between said rolls 4 and 5.
The heated roll fixing device of such a construction has such advantages that it is operated under relatively low electric power and that it has little or no chance of causing fire and the like.
Also, the heated roll fixing device employing a heat roll coated with silicone rubber as a heat-resistant surface lubricant generally has been widely used, because such device provides an appropriate contact width under relatively low electric power, and since the roll surface is soft, toner images on the supporting body are never crushed, whereby copies of high quality are produced.
However, since rubber material is thermally insulative to a high degree, it is necessary to increase the inner temperature (the temperature of internal heating means) to a considerable degree in order that the surface of the heat roll may have a desirable surface temperature.
This, however, exerts adverse influence on the connection or bond between the inner surface of the coated rubber layer and the outer surface of the core and at the same time it leads to the deterioration of the rubber material.
Such phenomenon is clearly seen when supplying offset preventing liquid such as silicone oil on the roll.
In order to reduce the difference between the inner temperature and the surface temperature, it is effective to reduce the thickness of the rubber layer as much as possible. However, there, of course, exists a limit in reducing the thickness, it is difficult to solve the aforesaid problem by doing this.
To overcome this, it has already been proposed to provide an outer heating means.
As an example which has already been devised, there is a heated roll fixing device, as shown in FIG. 2, of the type, in which an outer heat roll 8 (hereinafter referred to as a third roll), which is rotated in contact with the heated roll 4 and is maintained at high temperature, is provided for heating the heated roll 4.
However, with such a construction, since the third roll 8 has a large heat capacity, it is necessary to rotate the heated roll 4 even during the wait time between the production of a copy and the next copy (hereinafter referred to as pre-heat time) so that only a part of the heated roll 4 is not heated. In other words, the heated roll 4 is heated substantially uniformly by the third roll 8. Further, as the area of the rolls 4, 5 and 8 becomes large in total, the quantity of heat to be discharged also becomes large. In other words, the quantity of heat to be effectively discharged for cleaning is reduced with the result of increased heat loss. In addition, the toner particles which are offset on the heated roll 4 are likely to be transferred onto the third roll.
Another example which has already been proposed (devised) is shown in FIG. 3, wherein the heat roll 4 is provided on the surface thereof with an infrared ray lamp 9 so that said heated roll may be heated.
However, with such construction, there exists a fear of causing fire since the infrared ray lamp 9 has high temperature. This eliminates such an advantage of the heat roll fixing device that there is no fear of causing fire, and at the same time there occur irregularities in the surface temperature of the rolls, thereby resulting in an inefficiently heated roll fixing device.
It is therefore an object of the present invention to provide a novel heat roll fixing device for electrophotographic copying machines employing a heated roll coated with heat-resistant releasing agent, in which there are provided in combination an inner heating means, such as, an infrared ray lamp, and an outer heating means with small heat capacity, thereby maintaining various advantages of a heat roll fixing device as well as improving a heated roll fixing device which has already been proposed and is provided with an outer heating means.
Another object of the present invention is to provide a heated roll fixing device for electrophotographic copying machine, which is free from the above-described drawbacks as experienced in the known device.